The present invention relates to a valve unit for controlling the delivery pressure of a gas according to the preamble to the main claim.
As is well known, these units are used for controlling the delivery of gas to a burner or to another similar user so as to vary its delivery pressure in a controlled manner according to predetermined physical parameters, for example, measured by an electric current supplied to the solenoid of an electromagnetic actuator of the valve unit.
The control carried out by means of these units typically provides a characteristic curve in which, below a certain minimum current value I0 no gas-flow is delivered, between the value I0 and an intermediate value I1 a minimum gas-flow is delivered, and between the intermediate value I1 and a higher value I2 the delivery pressure is modulated up to a maximum pressure value.
The control or the cutting-off of the gas delivery up to the minimum current value I0 is carried out by means of an on-off gas valve, whereas the subsequent control of the delivery pressure between the minimum and maximum pressure values is carried out by means of a servo-valve controlled by a modulation unit with a diaphragm. The diaphragm of this unit is acted on, on one side, by the delivery pressure of the gas as detected in the delivery duct to the user and, on the other side, by a resilient load which is subjected to the action of the movable device of an electromagnet and is variable between a minimum value and a maximum value in dependence on the current piloting the electromagnet.
A problem encountered with these known units is that of the correct regulation of the minimum value of the gas pressure delivered. This value depends upon the minimum resilient load acting on the diaphragm which in turn depends upon the position in which a control rod of the electromagnetic actuator stops against a low-point abutment. Naturally, the regulation of the minimum pressure is more precise the lower is the elastic constant of the spring acting on the diaphragm. The selection of this spring is therefore subject to a compromise between the need for stiffness to provide an appreciable variation of the resilient load at the stage of the modulation of the delivery pressure up to the maximum value and the need for softness to ensure precision and stability of adjustment during operation at the low point. The relative stiffness in the modulation stage is nevertheless important for bringing about sufficient variation of the resilient load within the limited travel available for the movable device of the electromagnetic actuator which brings about the pressure variation.